Power cable

ABSTRACT

The geometric dimensions and power losses of a power cable are maintained constant along its length, notwithstanding changing ambient thermal conditions. This is made possible by changing the conductor material at different locations, for example, copper to aluminum, having different thermal resistivity. The sections of different conductor material are joined by conventional techniques such as welding, soldering, etc. Copper is used at the ends to simplify splicing.

United States Patent Hirsch et al. 1 May 30, 1972 [54] POWER CABLE3,263,193 7/1966 Allen ..333/99 s [72] Ihventors: Christian worm Hirsch,Lysakfr; J 3,317,651 5/1967 Demess 1 74/ 128 X Norman" Jflhnsen, Oslo,both of Norway FOREIGN PATENTS OR APPLICATIONS Assignee= InternationalStandard Electric p 1,006,573 4/1902 France 174 94 tion, New York, N.Y.

l [22] Filed: Oct. 20, 1970 OTHER PUBL CATIONS A 1 No 82 248 Dummer etal.,Wires&R.F. Cables, Pitman, 1968,p. 129

Primary Examiner-E. A. Goldberg [301 Foreign Application Priority DataAttorney-C. Cornell Remsen, Jr., Walter J. Baum. Paul W. Hemminger,Charles L. Johnson, .lr., Philip M. Bolton, Isidore Feb. 6, 1970 Norway..417/70 Togut, Edward Goldberg and Menom y Lombardini, 1 Nov. 8, 1969Norway ..4437/69 [57] ABSTRACT [52] U.S. Cl... ..174/128, 174/126 CP;174/130 6 [51] ...H0lb 5/08 The geomemc dlmenslons and Power losses of apower cable [58] Field 61 Search 174/126 R 126 C? 128 130 are maintainedalng its length withstanding I 174/110 R 25 R 333799 changing ambientthermal conditions. This is made possible by a changing the conductormaterial at different locations, for example, copper to aluminum, havingdifferent thermal resistivi- [56] References ty. The sections ofdifferent conductor material are joined by UNITED STATES PATENTSconventional techniques such as welding, soldering, etc. 2 992 9597/1961 5 h l 338/330 x Copper is used at the ends to simplify splicing.

c rewe 1us 3,094,679 6/1963 OConnor ..338/330 9 Claims, 5 DrawingFigures AL UMl/VUM 1 POWER CABLE BACKGROUND or THE INVENTION 1 Field ofthe Invention The present invention relates to power cables which areprefabricated to be installed along a certain cable route andparticularly to the maintenance of relatively constant propertiestherealong.

2. Description of the Prior Art The current carrying capacity of powercable is determined by factors such as the properties of the conductor,of the insulation and of the cable surroundings. The most critical ofthese factors are the properties of the insulation, in particular, itsabilities to withstand heat. The cable insulation will in most caseslose its original insulating properties if it is subjected to excessiveheat for any period of time. The heat affecting the insulation isdetermined by the conductor resistance and the heat dissipationproperties of the cable insulation and those of the surroundings. Aparticular cable installed at a particular place must therefore not beloaded to such an extent as to cause deterioration of the insulation. Inorder to be on the safe side, overdimensioning of cable conductors arequite common.

Power cables are usually designed so that the power losses i.e., lossesin the conductor, dielectric losses in the insulation, sheath lossesetc. are kept constant along the cable at constant current and constantvoltage. However, there are two factors which may cause highertemperature than desirable if measures are not taken for compensation ofsuch losses. First, the heat dissipation properties of the surroundingsmay vary considerably along a cable route, causing increased cabletemperature at places where the heat dissipation properties are poor.Second, the conductor temperature may increase at places such as jointswhere two cable lengths are joined together. Problems caused by thesetwo factors shall be considered in detail below.

The thermal resistivity of the surroundings depends on whether a cableis laid in sand, soil, clay, water, ducts etc. Such varying conditionsare taken into account when designing a cable installation, allowingheavier loading of a cable having good heat dissipation possibilitiesthan of one having poor heat dissipation. However, often the thermalresistivity of the surroundings will vary along the cable route. When insuch cases the same type of cable with the same dimensions, especially,the same conductor cross-section and material, is used along the wholecable route, the conductor temperature will vary along the cable. Sincethe maximum conductor temperature must not exceed a certain limitdepending on the type of the cable, the operating voltage etc., theseconditions must be taken into account during the dimensioning of thecable.

The part of the route having the poorest heat dissipation propertieswill therefore be the critical factor when determining the cabledimensions. As a result of this, the part of the cable passing throughareas with better heat dissipation properties will have a lowerconductor temperature than permissible, because these parts are notloaded to their full current carrying capacity. Such overdimensioning isusually very expensive. This is especially the case for long submarinecables, where most of the cable is exposed to very favorable thermalconditions namely the part of the cable which is lying in the waterwhile only a small part at each end of the cable is in the ground underrelatively poor thermal conditions and therefore being the criticalfactor for dimensioning the whole cable.

In order to avoid poor utilization due to variation of the heatdissipation properties along the cable route, it has been suggested toalter'the conductor cross-section by joining together cable lengths withdifferent cross-sections by means of special cable joints. Thisprocedure is normally elaborate and costly, particularly in the case ofan oil-filled cable. Further such special joints are normallyundesirable for technical and electrical reasons especially inconnection with submarine cable. Thus, however carefully a joint isdesigned and executed, the dielectric strength of the cable at or nearthe joint will in many cases be somewhat lower than that of theinsulation in the rest of the cable. Furthermore, due to the added wallof insulation required at a joint, the conductor may often run hotter inthe middle of a joint than in the rest of the cable.

In order to obtain the best possible cable installation, a cable shouldbe designed such that the operating temperature on the cable insulationat all places is just below the maximum allowable temperature. It might,however, be desirable, at joints, to specify a somewhat lowertemperature.

Conductors for insulated cables in the past normally have been made withcopper because of its high conductivity of electric current and lowpower loss capability. However, in more recent years, its high cost hasprompted a change to less expensive aluminum in spite of the fact thatits conductivity is only about 60 percent of that of copper. But thechange from copper to aluminum for certain types of applications, suchas cables for high voltages has been slow. This has been due to certaindisadvantages of aluminum as compared to copper, such as its higherthermal coefficient of expansion and more complicated procedure for thejoining of conductors.

For cables which are to be directly buried, the above disadvantages arenot of significance, since a buried cable is confined so that theheating of the cable during loading cannot result in a longitudinal ortransverse movement of the cable.

Also in the case of submarine cables the problem is minor, since theportion of the cable which is submerged will normally heat up verylittle due to the efficient cooling provided by the water, while theshore ends normally are directly buried.

For cables installed in ducts the condition are quite different in thatthe cable is allowed to move longitudinally in the ducts during heatingand cooling. To take up this expansion it is normally necessary toprovide underground chambers or manholes at each splicing point in orderto allow the cable to be bent in the shape of a U" on each side of thesplice. This practice has been used for many years in the case of cableswith copper conductors and methods have been found to join the twocopper conductors in a manner which, from experience, has proven to makethe conductor stand this repeated bending and tensioning. The method ofjoining the two copper conductors normally consists of placing a coppersleeve over the conductors and compressing this sleeve by means of ahydraulic press, the sleeve thus providing the necessary support to helpthe conductors stand the repeated bending.

The above joining procedures has proven to be less reliable in the caseof aluminum conductors because of the lower tensile strength of analuminum sleeve together with the before mentioned greater coefficientof thermal expansion of aluminum. Joints in aluminum conductors for hightension cables are therefore normally made by welding or soldering.While this procedure has proven entirely satisfactory for cablesdirectly buried for the reasons given above, the softening of thealuminum metal at the weld may make this procedure less suitable in thecase of a cable installed in ducts. With the conductor stranded fromhalf-hard or three quarters hard aluminum wires, the soft area at thejoint will constitute a weak spot when the cable is subjected to thebending cycles described above.

SUMIVIARY OF THE INVENTION An object of the present invention is toprovide a more efficient cable by preventing the overdimensioning whichfollows from using conventional dimensioning methods and to avoidundesirable extra joints. The overdimensioning problem may be taken careof by changing the electrical resistance of the cable conductor inaccordance with the thermal resistivity along the cable route. Thethermal resistivity or heat dissipation properties should be measuredalong the cable route, or estimated, at the early planning stage inorder to decide which conductor resistance value should be used on thevarious parts of the route in order to ensure optimum utilization of thecurrent carrying capacity on all parts of the cable.

Another object of the present invention is to provide a power cablecomprising a conductor made wholly or partly of aluminum, designing thecable lengths such that the temperature at the joints between succeedinglengths is kept aslow as possible. This is obtained according to thepresent invention by gradually changing the aluminum portion of theconductor cross-section to copper at the ends of each length.

According to the present invention, the cable conductor is designed suchthat the electrical resistance per unit length of the conductor, at anypoint along the cable route, is chosen in accordance with the desiredconductor temperature at that particular point. The required change ofthe electrical resistance of the conductor is obtained by changingpartly or wholly from one conductor material to another and vice versain the longitudinal direction of the conductor. The conductor materialsare preferably aluminum and copper and the change of the electricalconductor resistance is achieved while maintaining the geometricaldimensions of the conductor.

By designing a power cable in accordance with the present invention, theconductor temperature within the joint will be much lower than it wouldotherwise be. This again means that the insulation in and near the jointwill operate at a lower temperature than the insulation inthe remainingpart of the cable. In most cases this will result in lower dielectriclosses. An additional advantage is that the problem of joining twolengths of cable together is greatly reduced, since it is easier to jointwo copper conductors than it is to join two made of aluminum.

It is a feature of the invention that the conductor resistance along acable route where the thermal resistivity of the cable surroundings varyis changed in accordance with the change of thermal resistivity of thecable surroundings by changing the conductor material such that the,geometrical dimensions of the cable is kept constant throughout thecable route.

By designing a power cable according to the present invention oneobtains optimum utilization of the current carrying capacity on allparts of the cable without using cable sections of different dimensionsand special joints between such sections. I

The present invention avoids the mentioned disadvantages of usingaluminum as conductor material, while it maintains the savings which arepossible by using this conductor material instead of copper. In thiscaseof duct installed cables only a few per cent of the cable length need tohave copper as conductor material.

The above mentioned and other features and objects of the presentinvention will clearly appear from the following description ofembodiments of the invention taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. la, lb and show cross-sectionsof a conductor where the conductor materials in the various layers arechanged gradually from aluminum to copper,

FIG. 2 is a longitudinal section through a conductor along the conductoraxis showing the different cross-sections as illustrated in FIGS. 1a, lband 1c, and

FIG. 3 shows six conductor wires of one layer where the conductormaterial of the wires is changed over a certain length.

DETAILED DESCRIPTION OF THE INVENTION In FIGS. 1a, 1b and 1c are shown apower cable conductor consisting of a hollow tubular core 1 and twolayers 2 and 3 of a plurality of strands of profiled wires. In FIG. 1a,the tubular core 1 and the inner profiled layer 2 are made of aluminum,while the outermost layer 3 is made of copper. In FIG. lb the conductormaterial of the inner layer 2 has been changed to copper, while in FIG.10 the tubular core 1 is also made of copper. The cross-sections shownin these three figures represent power cable conductors, the conductormaterial of which is chosen in accordance with a desired temperature onthe conductor.

The change in conductor material is preferably obtained by designingconductors which in the longitudinal direction is constituted by wiresof at least two different conductor materials.

For large cables the conductor is usually designed as a multistrandconductor, the number of wires constituting the conductor and thegeometrical cross-section and shape of these wires are maintainedthroughout the cable route.

When the conductor is a multistrand conductor the desired changes of theconductor resistance is obtained by changing the conductor material inat least one of the wires. The desired change from one conductormaterial to another is obtained by splicing wires of different conductormaterials together by welding, soldering, compression joining or similarprocesses prior to working of the wire or conductor to its finalcross-section and shape. An advantage of this is that the joint isautomatically tested during the final working of the cable, forinstance, during the drawing of the cable.

Alternatively, a short piece of wire made of one of the two metals maybe spliced with a short piece made of the other metal in advance, ifnecessary under laboratory conditions to produce good quality joints, sothat only joints between wires of similar metals have to be made duringthe stranding operation.

Another variation of the invention is that in any cross-section of thecable comprising aluminum wires as well as copper wires, the aluminumwires are arranged in the core and in the inner layers while the copperwires are arranged in the other layers.

In FIG. 2 is shown a cut through a conductor similar to that shown inFIGS. 1a-1c, with sections A-A, B-B and C-C corresponding to those ofFIGS. la, lb and 10 respectively. At the left the complete conductor ismade of aluminum, while at the right the whole cross-section is changedto copper. In this figure it is shown that the conductor material is notchanged abruptly in the whole conductor cross-section. Layers 4, 5 and 7indicate the aluminum portions of the outermost layer, the innerprofiled layer and the tubular core respectively, while 6, 8 and 9indicate the copper portions.

When the conductor consists of two or more wires each with a joint oftwo different metals it is preferable to space the joints of the twometals evenly over a certain length of the conductor in order to avoidabrupt changes of the mechanical and electrical properties of the saidconductor. Preferably the distance between each joint should be at least10 cm.

In FIG. 3 is shown six of the plurality of profiled wires representingsix of the 18 wires on the outer multistranded profiled wire layer ofthe conductor similar to that shown in FIGS. la, lb, 1c and FIG. 2. hisconsidered advantageous to undertake the change from one conductormaterial to another over a certain length, and in FIG. 3 is illustratedhow this may be effected in individual strands. At the extreme left allwires are of aluminum, while at the extreme right all wires are ofcopper. As will be seen from the drawing each wire joint is placed at acertain distance from the neighboring joints, thereby obtaining aninterleaved pattern. The joint of wire 13 is placed between unjointedportions of the neighbor wires 12 and 14.

Another factor which will help in spreading the wire joints over acertain length is the cable stranding effect. If all joints 1 werealigned on the wire pay-off equipment the joints will appear staggeredwhere the wires are stranded on to the hollow core or onto a lowerlayer.

In the case of long oil-filled submarine cables which are manufacturedin discrete lengths and jointed together in the factory, it will also beadvantageous to change to copper just before a joint. An advantage isthat a mechanically stronger joint is obtained. This is of particularimportance in connection with pipetype cable and duct cable.

In the case of pipe-type cables, which normally consist of severallengths joined together, it is very important that the tensile strengthof the joints is not lower than that of the cable. The mechanical strainon the pipe-type cable is considerable when the cable expands andshrinks within the confinement of the pipe due to temperaturevariations.

Regrading duct cables, lengths of which are joined together in specialmanholes, the change to copper should preferably be undertaken at pointswhere the cable is still within the duct. Thereby the whole portion ofthe cable which is subjected to the most severe bending stresses, e.g.,the portion located within the manhole, has copper as a conductormaterial and the many years of experience gained with duct cables withcopper conductors may be applied also to an aluminum/copper conductorcable. The change to copper should however, be made close to theentrance of the manhole, for example, one meter within the duct, inorder to use the least possible amount of copper.

It is also of advantage to change to copper near a joint even when thecable is not installed in a duct or pipe and subjected to the addedmechanical strain associated to these types of installation. This isparticularly true in the case of submarine cable since these joints mayhave to be made under less than ideal conditions, such as on board aship, and the quality of the joint therefore may not be the very best.For submarine cables which are subjected to unusual strain, eitherduring laying (deep water) or later, the added strength of the copperjoint, may also warrant the adoption of this conductor construction.

By keeping the geometrical dimensions of the cable constant throughoutthe cable route, the manufacturing process is facilitated. Once theconductor has been made, it may be passed through the furthermanufacturing steps, such as insulating, sheathing and armouringprocesses, in one pass.

It should be noted that the patterns shown in the drawings are merelyfor illustrations of the appearance of the conductor. Many other changesmay be made in the design and configuration of the cable withoutdeparting from the spirit and scope of the present invention.

What is claimed is:

1. A power cable subjected to varying thermal characteristics along aroute comprising a plurality of adjoining parallel longitudinal.electrical conductors each having longitudinal sections of aluminum andcopper jointed in series, said sections each having a predeterminedlength and disposition selectively matching the changesin the thermalcharacteristics along said route to maintain said conductors below apredetermined maximum temperature and to minimize power loss therealong,and said conductors having constant crosssectional dimensions alongtheir length.

2. A power cable according to claim 1, wherein a substantial portion ofa length of each said conductor is made of aluminum and the end portionsthereof are made of copper.

3. A power cable according to claim 2, wherein said conductors aremultistrand conductors arranged in layers.

4. A power cable according to claim 3, wherein said plurality ofconductors include the same number of wires throughout the cable length.

5. A power cable according to claim 4, wherein the geometricalcross-section and shape of each of the wires are substantially uniformthroughout the cable length.

6. A power cable according to claim 5, wherein the crosssection of thecable comprises aluminum wires and copper wires selectively arranged ininner and outer layers along different longitudinal sections to providegradual changes of material along the length of cable.

7. A power cable according to claim 6, wherein said aluminum wires arein inner layers and said copper wires in outer layers.

8. A power cable according to claim 6, wherein the joints of adjacentwires in a multistrand layered conductor are spaced apart within apredetermined length of the cable.

9. A power cable according to claim 8, wherein said joints of adjacentwires around the circumference of said cable are stagger-ed to providean interleaved pattern along the length of the layer.

2. A power cable according to claim 1, wherein a substantial portion ofa length of each said conductor is made of aluminum and the end portionsthereof are made of copper.
 3. A power cable according to claim 2,wherein said conductors are multistrand conductors arranged in layers.4. A power cable according to claim 3, wherein said plurality ofconductors include the same number of wires throughout the cable length.5. A power cable according to claim 4, wherein the geometricalcross-section and shape of each of the wires are substantially uniformthroughout the cable length.
 6. A power cable according to claim 5,wherein the cross-section of the cable comprises aluminum wires andcopper wires selectively arranged in inner and outer layers alongdifferent longitudinal sections to provide gradual changes of materialalong the length of cable.
 7. A power cable according to claim 6,wherein said aluminum wires are in inner layers and said copper wires inouter layers.
 8. A power cable according to claim 6, wherein the jointsof adjacent wires in a multistrand layered conductor are spaced apartwithin a predetermined length of the cable.
 9. A power cable accordingto claim 8, wherein said joints of adjacent wires around thecircumference of said cable are stagger-ed to provide an interleavedpattern along the length of the layer.